You might be surprised to find out that NONE of the songs I have written ended up sounding like what I was originally hearing in my head at the very beginning.
I often start working on a song by picking the style first. I will sit and simply audition some of the styles. I apply them to my chord progression and see what works and what sounds good and what fits the feeling, and do any of them inspire the "magic?" I like to work in the rock and country genre for the songs I write. Simply varying the tempo will change the feel of the song. Normally, I have my Taylor acoustic and I'm playing along searching for the groove.
I call it the evolution of my song. What I hear in my head is the seed. As I play with the various styles and genre's , keys, and tempos, eventually something starts to take root. I settle on a style and go from there. I also make use of my DAW and the ability I have in it to do creative editing to add the little unique things that set that song apart from the ordinary.
I do that too. I call it developing a song and think it to be similar to what a professional songwriter who writes daily would do for inspiration. Unfortunately, I think that is the exact opposite of the original poster dga's problem. Like him, I sometimes get an inspiration for a song from another source other than BIAB/RB/DAW. In fact, over the years as I wrote songs, my songs were written either on guitar or simply in my head. I've hummed dozens of songs into cassette and voice recorders. I use to carry one with me at all times. I wrote the song first and then made a demo of that written song. That changed in 2013 with my purchase of BIAB.
All of the benefits of BIAB reversed that and BIAB began to steer the direction of songs created in my head. It became harder with all the BIAB styles and instruments to keep that original melody, chord progression and lyrics in my head. Many times my original idea was lost and morphed into something completely different.
It was my understanding of dga's problem that he to loses his original thoughts to BIAB suite of tools.